642 LECTURE XXIII. 



oosphere, travels towards the middle of the sac where it fuses 

 with one of the chalazal nuclei, which has likewise travelled 

 towards the middle of the sac, to form the definitive nucleus of 

 the embryo-sac. It may be suggested that the division which 

 leads to the formation of the nucleus of the oosphere and of 

 the so-called polar nucleus, is the one which we are seeking ; 

 in that case the so-called polar nucleus would be the polar 

 body. 



There remain yet a few cases to be considered in this con- 

 nexion, cases which are of somewhat doubtful nature, but 

 which nevertheless seem to have the same physiological signi- 

 ficance as those which we have just been discussing. In 

 these, the sexual organ undergoes complete cell-division as a 

 preliminary to the sexual process. Thus, in the Mucorini, 

 the apical portion of each of the conjugating hyphae is cut off 

 by a cell-wall from the remainder, and it is these two portions 

 which coalesce. Again, in Sirogonium sticticum, one of the 

 Zygnemese, as described by de Bary, each of the two con- 

 jugating cells undergoes division so as to form two or more 

 sterile portions and a fertile portion. These cell-divisions 

 may be fairly compared with those already described as 

 taking place in the pollen-grains of Phanerogams, and, in- 

 asmuch as they lead to the distinction of vegetative and 

 generative portions in the sexual reproductive organ, they 

 may be regarded as being of the same physiological signi- 

 ficance. It may be that the cell-divisions which take place in 

 the oogonium of the Characeae and lead to the formation of 

 the sterile "Wendungszellen" (Braun) ought to be included 

 here. 



We have now ascertained that the development both of 

 spores and of gametes is marked, in very many cases, by 

 certain peculiarities having essentially this result, that the 

 original nuclear substance of the reproductive organ, whether 

 it be a sporangium or a gametangium, does not all go to form 

 the nuclear substance of the one or more reproductive cells 

 produced by the organ, but that a portion of the original 

 nuclear substance is excluded, at some stage or other, from 

 forming part of the reproductive cell or cells. 



